A coil, Cook would later explain, is the heating element of an e-cigarette, known variously as a vaporizer, vaping pen, or by regulators, a nicotine delivery product. Heat from the coil converts liquid nicotine into a vapor — not smoke — which users inhale, or “vape.” The coil is powered by a battery and controlled by an atomizer — the electronics of an electronic cigarette.

Sometimes components wear out, and Donovan and her mother, Cheryl, of Wakefield, had come to Cumulus Vapors in Melrose for new ones.

Cheryl Donovan had smoked for 30 years, she said, for much of that time nearly two packs a day. Three months ago, after a friend quit smoking for vapor pens, and with a granddaughter on the way, she gave it a try. “I haven’t touched a cigarette since,” she said. She got her boyfriend, a smoker, to switch. Her daughter, too.

Across the region, converts to vaping share similar stories. A pack a day for 10 years. Two packs for 20 years. Forty years. And then a month ago, a year ago, two years ago: The last cigarette. Amazement.

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The addiction remains. Users can control the level of nicotine and, proponents argue, gradually reduce it to zero. But interviews with more than a dozen vapor users and public health officials in Melrose, Reading, Wakefield, and Lynn suggest that zero is a hard target.

While liquid nicotine, or e-juice, comes in a staggering array of flavors, from strawberry peach to snickerdoodle, the key ingredient is the drug, and few cut out nicotine completely.

“It’s nicotine and it’s an addictive drug, whether you’re smoking it in tobacco form or e-cigarette form,” said Ruth Clay, director of the joint Health Department of Melrose, Reading, and Wakefield.

While the e-cigarette industry is booming — estimates put revenues near $2 billion in 2013, a figure predicted to double this year — trends in the Northeast have trailed the West Coast and southern United States, retailers say. North of Boston, the landscape is changing fast as entrepreneurs open or seek to open e-cigarette stores in Lynn, Melrose, Reading, Saugus, Wakefield, and other communities.

The industry has outpaced state and federal regulators. Legislation before the Massachusetts House of Representatives would extend existing tobacco laws — including bans on sales to minors and smoking in public buildings — to nicotine delivery devices. First proposed last year, the bill now awaits release from the Joint Committee on Health Care Financing.

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“Some people find the pace frustrating, but we have a responsibility to be deliberative,” said state Representative Jason Lewis, Democrat of Winchester, a member of the committee and a cosponsor of the bill. “Cities and towns sometimes can act more quickly than the state.’’

‘We’ve only been open seven months and we’re already opening up number two.’

So local governments are crafting their own rules, often along the same lines as the proposed state law but sometimes going further. Public hearings to consider raising the minimum age to buy tobacco and nicotine delivery products from 18 to 21 were held earlier this week in Winchester and Wakefield; Reading has scheduled a hearing for 5:30 p.m. Thursday at Town Hall, and Melrose has its hearing slated for 8 p.m. Thursday at the City Hall, lower level.

Health officials also met on Tuesday in Stoneham to consider updating town tobacco regulations to include nicotine delivery products. “In Stoneham, a child can buy an e-cigarette,” said Maureen Buzby, coordinator of the Mystic Valley Tobacco/Alcohol Program.

“I applaud and support the efforts of local communities in restricting sales to minors,” Lewis said. “That can only be helpful in building momentum.”

Proponents say e-cigarettes perform the same essential function as tobacco cigarettes but with fewer toxins, and help people who are trying to quit smoking. Skeptics say the devices are unregulated, have health risks that are not yet understood, and may entice teenagers to get in the habit of smoking.

In Lynn, four friends opened MassVapors in October. The store has grown so quickly that the partners are working to open a second location in Boston next month. “We’ve only been open seven months and we’re already opening up number two,” said partner Viet Huynh, 35, of Beverly. “It’s a lot of money. It’s been so successful.”

On a recent Sunday, the video feed in the partners’ office showed six customers — five men and a woman, most appearing to be in their 30s — crowded around display counters. Huynh said MassVapors is one of the largest in the region. And, at seven months old, it was also one of the first.

Two weeks after the Lynn store opened, Larry Blauvelt, 51, and his brother Steve, 52, of Stoneham, opened EZ Cig Vapors in Wakefield. They picked the date — Oct. 18 — because it was the birthday of their late father, a lifelong smoker who died of lung cancer in 2001. After discovering vaporizers, the Blauvelts had quit smoking and found their mission, they said.

“We’ve had a tremendous amount of success stopping people from smoking,” said Larry Blauvelt. “I’d say 80 percent success, and the other 20 percent don’t want to stop.”

In Reading, David Mattuchio, 44, of Tewksbury, said he has met some resistance as he works to open his store, Vapor Station, next month. “It’s been adversarial in many ways, largely because of lack of information in the public,” he said.

“I have to educate everybody I encounter, from Board of Health inspectors to town planners to potential landlords. There are few people who know what this product is.”

Back at Cumulus Vapors, the Donovans, mother and daughter, were also looking for new flavors of liquid nicotine, which is sold in refillable or replaceable cartridges.

Cheryl Donovan preferred French vanilla.

“When you add a new cartridge, take a few dry hits to prime it,” Cook told her. “Otherwise you’ll get a burned taste.”

“I’ve gotten that so many times,” Cheryl Donovan said.

“Prime it,” Cook said. “You’ll see the difference.”

“You’re so helpful,” Nicole Donovan said. “The first place just handed us the stuff.”

The Donovans paid and walked out of Cook’s store into the sunlight, and fresh air.